(1) Field
The disclosed subject matter generally relates to reducing an amount of mercury discharged to an environment incident to the combustion of a fuel source containing mercury or mercury containing compounds, and more particularly to reducing an amount of mercury discharged in a combustion flue gas that is subjected to a flash dryer absorber (FDA) system.
(2) Description of the Related Art
Combustion of fuel sources such as coal produces a waste gas, referred to as “flue gas” that is to be emitted into an environment, such as the atmosphere. The fuel sources typically contain sulfur and sulfur compounds, which are converted in the combustion process to gaseous species, including sulfur oxides, which then exist as such (otherwise known as “acid gases”) in the resulting flue gas. The fuel sources typically also contain elemental mercury or mercury compounds, which are converted in the combustion process to, and exist in the flue gas as, gaseous elemental mercury or gaseous ionic mercury species (generally referred to hereinafter as “mercury containing compounds”).
Accordingly, flue gas contains dust, fly ash, and noxious substances such as acid gases, as well as other impurities, that are considered to be environmental contaminants. Prior to being emitted into the atmosphere via a smoke stack (“stack”), the flue gas undergoes a cleansing or purification process.
In coal combustion, flue gas often undergoes a desulfurization process, which typically occurs in a flue gas desulfurization system. There are several types of desulfurization systems, including wet flue gas desulfurization (WFGD), also known as “wet scrubbers” and dry flue gas desulfurization (DFGD), also known as “dry scrubbers.” There are two separate types of DFGD, the first is a spray dryer absorber (SDA), while the other is a flash dryer absorber (FDA).
Acid gases are removed from flue gas using a FDA system by chemically reacting a moist pulverous material with the acid gases contained within the flue gas. Generally, the acid gases are absorbed by the moist pulverous material, which is then separated from the flue gas by the particle separator. The moist pulverous material typically includes 0.5-5 wt. % water based on the total weight of the moist pulverous material and a basic reagent that will interact with contaminants to remove them from the flue gas. Examples of basic reagents that are useful in the moist pulverous material include, but are not limited to, particulate material collected from the flue gas (such as dust and fly ash), as well as alkaline material, which generally can be selected from lime, limestone, calcium hydroxide and the like and combinations thereof.
Recently, there has been an increased focus on the removal of mercury. Presently, there are various methods for removing mercury from flue gas emissions. Those methods include, but are not limited to the following: addition of oxidizing agents in a boiler upstream of the flue gas emission control system and then removing it with wet scrubbers; addition of reactants to bind mercury and remove it from the flue gas; and utilization of particular coal or fuel that minimizes the amount of mercury released when the coal or fuel is burned.
It has been shown that a number of generally known methods of mercury removal are effective to produce mercury salts, which can be dissolved and removed by the aqueous alkaline slurry used in a wet flue gas desulfurization system (WFGD). Some of these methods include the addition of halogen or halogen compounds, such as bromine, to the coal or to the flue gas upstream of the wet scrubbing operation, to provide oxidation of elemental mercury to ionic mercury and formation of mercury salts, which are then dissolved in the aqueous alkaline slurry incident to the sulfur oxide removal processes. However, the removal of mercury in a DFGD system has proven to be difficult to control and it is not easily predicted when designing a flue gas cleaning system with respect to mercury removal. The desired emission guarantee levels are often as low as 1 μg/Nm3 of mercury, which corresponds to a very efficient mercury removal in the DFGD system.